compress_ops: add magic and look for compress_plugin_mops symbol

We need more validation of passed ops structure to allow
further API changes in future.

This change looks for compress_plugin_mops symbol in the
dynamic plugin library to make sure that the new magic
and new get_tstamp64 members are handled correctly.

Link: alsa-project#29
Link: alsa-project#30
Fixes: 0bd5530 ("compress_ops: add get_tstamp64")
Signed-off-by: Jaroslav Kysela <perex@perex.cz>

Author: perexg

include/tinycompress/compress_ops.h

@@ -8,6 +8,8 @@
 #include "sound/compress_offload.h"
 #include "tinycompress.h"
 
+#define COMPRESS_OPS_V2		0xadcc0002	/* version 2 magic */
+
 /*
  * struct compress_ops:
  * ops structure containing ops corresponding to exposed
@@ -16,6 +18,7 @@
  * done in compress_hw.c
  */
 struct compress_ops {
+	unsigned int magic;	/* version of this structure */
 	void *(*open_by_name)(const char *name,
 			unsigned int flags, struct compr_config *config);
 	void (*close)(void *compress_data);

src/lib/compress.c

@@ -58,6 +58,7 @@
 #include <dlfcn.h>
 #include <stdio.h>
 #include <string.h>
+#include <sys/errno.h>
 #include <sys/time.h>
 #include "tinycompress/tinycompress.h"
 #include "tinycompress/compress_ops.h"
@@ -136,14 +137,20 @@ static int populate_compress_plugin_ops(struct compress *compress, const char *n
 		return ret;
 	}
 
-	compress->ops = dlsym(dl_hdl, "compress_plugin_ops");
+	compress->ops = dlsym(dl_hdl, "compress_plugin_mops");
 	err = dlerror();
 	if (err) {
 		fprintf(stderr, "%s: dlsym to ops failed, err = '%s'\n",
 				__func__, err);
 		dlclose(dl_hdl);
 		return ret;
 	}
+	if (compress->ops->magic != COMPRESS_OPS_V2) {
+		fprintf(stderr, "%s: dlsym to ops failed, bad magic (%08x)\n",
+				__func__, compress->ops->magic);
+		dlclose(dl_hdl);
+		return -ENXIO;
+	}
 	compress->dl_hdl = dl_hdl;
 	return 0;
 }

src/lib/compress_hw.c

@@ -641,6 +641,7 @@ static int compress_hw_set_codec_params(void *data, struct snd_codec *codec)
 }
 
 struct compress_ops compress_hw_ops = {
+	.magic = COMPRESS_OPS_V2,
 	.open_by_name = compress_hw_open_by_name,
 	.close = compress_hw_close,
 	.get_hpointer = compress_hw_get_hpointer,